1. Field of the Invention
The present invention relates to a condenser microphone unit and a condenser microphone including the condenser microphone unit, the condenser microphone unit enhancing the sensitivity and the S/N ratio of the condenser microphone.
2. Related Background Art
Japanese Unexamined Patent Application Publication No. 2008-098851 discloses a condenser microphone that includes a condenser microphone unit having a diaphragm that vibrates in response to sound waves and a fixed electrode (also referred to as “back electrode”) which are disposed oppositely with a spacer therebetween to configure a capacitor having a variable capacitance in response to vibration of the diaphragm. Such a condenser microphone unit is composed of built-in components, including the diaphragm and the fixed electrode, accommodated into a unit casing.
FIGS. 4 and 5 are each a cross-sectional view illustrating a typical conventional condenser microphone unit 10. FIG. 4 illustrates a cylindrical insulating washer, whereas FIG. 5 illustrates an insulating washer having an increased internal diameter at its upper end.
The condenser microphone unit 10 includes a unit casing 12 that accommodates a diaphragm 14 held by a diaphragm holder 13, a spacer 15, a fixed electrode 16, an insulating washer 17, a cylindrical electrode 18, and a circuit board 19 on which electric components including an FET 19a are mounted, in sequence. The open end 12a of the unit casing 12 is bent inward to fix the built-in components in the unit casing 12 to the interior of the unit casing 12 such that the circuit board 19 is urged toward the bottom 12b of the unit casing 12.
In the condenser microphone unit 10 having such a configuration, a capacitor is defined by the opposing diaphragm 14 and fixed electrode 16. Vibration of the diaphragm 14 in response to received sound waves leads to variations in the gap between the diaphragm 14 and the fixed electrode 16 and thus the capacitance of the capacitor. The variable capacitance is output as audio signals due to a change in voltage.
The insulating washer 17 ensures insulation between the fixed electrode 16 and the unit casing 12. The insulating washer 17 also fixes the radial positions of the components fitted in the condenser microphone unit 10, thereby preventing eccentricity of the components.
After the open end 12a of the unit casing 12 is bent as described above, however, the positions of the built-in components are fixed by the urging force exerted on the circuit board 19, the cylindrical electrode 18, the fixed electrode 16, the spacer 15, the diaphragm 14, the diaphragm holder 13, and the bottom 12b of the unit casing 12. Furthermore, the fixed electrode 16 is insulated from the unit casing 12. Thus, the insulating washer 17 becomes unnecessary.
The insulating washer 17 is composed of a resin material, such as polycarbonate or ABS. Although such a resin material generally has an extremely high volume resistivity, the surface resistivity decreases with an increase in humidity in the surrounding air. In the case where the shortest distance along the surface of the insulating washer 17 (creepage distance) between the fixed electrode 16 and the unit casing 12 is short, increased humidity causes a leakage current causing noise on the surface of the insulating washer 17. Insulation by dry air does not cause such a leakage current.
In addition, the diaphragm vibrates in response to a difference between pressures exerted on the front and rear surfaces of the diaphragm in the condenser microphone. As the volume of an air chamber increases in the rear of the diaphragm (on the fixed electrode side), the back pressure generated by vibration of the diaphragm decreases. This facilitates vibration of the diaphragm, thus enhancing the sensitivity and frequency response in bass sound of the condenser microphone. The insulating washer 17, however, which reduces the volume of the air chamber, precludes vibration of the diaphragm, thus lowering the sensitivity and frequency response in bass sound of the condenser microphone. Removing the insulating washer 17 for utilization of the corresponding volume for the air chamber can enhance the sensitivity, S/N ratio, and frequency response in bass sound compared to a condenser microphone unit having the same external shape.
In the case where the cylindrical electrode 18 connecting the fixed electrode 16 and the FET 19a and the insulating washer 17 surrounding the cylindrical electrode 18 are concentrically disposed, the condenser microphone unit has stray capacitance (floating capacitance) in proportion to the relative permittivity, i.e., 2 to 3 of the plastic, compared to the case of insulation by air which has a relative permittivity of approximately 1. The stray capacitance lowers the sensitivity of the condenser microphone.